It does boot them if they don't actually do something on their controller after a certain time. However, an option for people to manually boot someone would only be abused by countless trolls kicking people for no real reason. Getting one more person to agree to kick someone when you suggest it isn't that hard. Other online games usually have larger parties when 'dungeons' occur, so it is a safer option. One person might suggest someone gets kicked, but unless they deserve it, you're likely to have at least one decent person that won't agree in the party when there are larger numbers. The small 3 man fire team keeps this from ever being a viable option.